Optical fibers and fiber gratings are useful for telecommunication transmission and networking. Basically, optical fibers are thin strands of glass capable of transmitting information-containing optical signals over long distances with low loss. In essence, an optical fiber is a small diameter waveguide comprising a core having a first index of refraction surrounded by a cladding having a second (lower) index of refraction. As long as the refractive index of the core exceeds that of the cladding, a light beam propagated along the core exhibits total internal reflection, and it is guided along the length of the core. Typical optical fibers are made of high purity silica, and various concentrations of dopants may be added to control the index of refraction.
Optical gratings are important elements for selectively controlling the paths or properties of traveling light. Gratings based on optical fibers are of particular interest as components in modem telecommunication systems. For example, in long-distance transmission of optical signals, the accumulation of signal dispersion may be a serious problem. This problem intensifies as the signals travel longer distances or with an increase in the number of channels in a wavelength-division-multiplexed (WDM) optical communication system. Efforts to compensate for chromatic dispersion to date have involved use of dispersion compensating gratings, which may be used in combination with dispersion compensating fibers. See M. I. Hayee et al., IEEE PHOTONICS TECHNOLOGY LETT., Vol. 9, No. 9, p. 1271 (1997); R. I. Laming et al., IEEE PHOTONICS TECHNOLOGY LETT., Vol. 8, No. 3 (1996); W. H. Loh et al., IEEE PHOTONICS TECHNOLOGY LETT., Vol. 8, No. 7 (1996); K. O. Hill et al., OPT. LETT., Vol. 19, p. 1314 (1994); and U.S. Pat. No. 5,701,188 issued to M. Shigematsu et al., on Dec. 23, 1997, incorporated herein by reference. The above-mentioned dispersion compensating devices, however, are not flexible and provide only a fixed degree of compensation for chromatic dispersion. More flexible designs are desired, as active control of dispersion is important for high speed systems.
Several designs for in-fiber tunable dispersion compensating elements based on chirped Bragg gratings have been described. A chirped grating may be obtained by applying an external perturbation-generating field (an "external gradient") non-uniformly along the length of the fiber, resulting in non-uniform changes in properties of the fiber grating and a chirp. Use of a temperature gradient as an external gradient to impose a chirp on a fiber grating is described, for example, in U.S. Pat. No. 5,671,307 to Lauzon, issued Sep. 23, 1997, which is incorporated herein by reference. Similarly, it was proposed that a chirp could be induced in a grating using a strain gradient in P.C. Hill & B. J. Eggleton, ELECT. LETT. Vol. 30, 1172-74 (1994). A device involving the etching of the outer surface of the fiber to produce a taper for providing a chirp in the fiber grating region is described in M. A. Putnam et al., "Fabrication of Tapered, Strain-Gradient ChirpedFiber Bragg Gratings," ELECT. LETT. Vol. 31 (1995), at p. 309, also incorporated herein by reference. These etched devices have disadvantages in that hydrofluoric acid is used during fabrication to etch the fiber surface, and the resultant fiber is fragile in that it has significant parts of its cladding etched away.
As may be appreciated, those concerned with technologies involving optical communications systems continue to search for new designs enabling for more flexible methods for providing chirped gratings and compensating for chromatic dispersion. It is desirable to have an optical grating device that may be used as a tunable dispersion compensator whose characteristics and performance may be selectively altered, that does not require a continuous use of power, and that does not require processing methods which weaken the stability of the fiber.